An electret is a dielectric material having a quasi-permanent electric charge or dipole polarization. Electret materials are quite common in nature. For example, quartz and other forms of silicon dioxide are naturally occurring electrets. However, many electrets are synthetic, prepared by heating a dielectric above its melting temperature and then cooling the melted dielectric in a strong electric field.
In certain examples, electret material can be used as a membrane for a microphone. An electret condenser microphone (ECM), for example, eliminates the need for a polarizing power supply in a digital microphone system by providing a permanently charged or polarized material. However, ECMs can be sensitive to temperature.